Sodium carbonate monohydrate vs anhydrous

Photopathe · Jul 30, 2020

Hi!
In developer recipes (such as Ansco 130) where sodium carbonate monohydrate is called for, may one use the cheaper and more common sodium carbonate anhydrous as a substitute ? Monohydrate is 15% water so i was thinking about using 85% anhydrous + 15% water instead of the original mass of monohydrate. But maybe there is more to it in terms of chemical interactions?
Thanks!

JPD · Jul 30, 2020

If you have anhydrous Sodium carbonate and the recipe calls for the monohydrate. Multiply by 0,833.

Recipe: Sodium carbonate 100g = anhydrous 83,3g

Photopathe · Jul 31, 2020

Thanks!
And complete the missing part with water?

JPD · Jul 31, 2020

Photopathe said:
Thanks!
And complete the missing part with water?

No extra water. Just start with 750 ml varm water, mix the chemicals in the correct order and then fill up to 1000 ml.

Photopathe · Jul 31, 2020

Many thanks! You have been most helpful.

Donald Qualls · Jul 31, 2020

Important clarification, since something up above could be read backward: if the recipe wants anhydrous, you have to use more monohydrate -- you're adding the required amount of sodium carbonate, plus the water of hydration it includes. So, if it's 15% water by mass, you need to use 115% as much by mass as the recipe calls for, not 83%.

koraks · Jul 31, 2020

Photopathe said:
the cheaper and more common sodium carbonate anhydrous

The anhydrate is neither cheaper nor more common. In fact it's relatively costly and since it's hygroscopic (it will absorb moisture from the air to form monohydrate), it's also not perfectly stable.
Are you sure you're not confusing anhydrous with decahydrate? The latter is indeed cheap and quite common as the cleaning soda sold in supermarkets.

JPD · Jul 31, 2020

From an earlier discussion, that I thought I should include in case this thread comes up in searches: https://www.photrio.com/forum/threads/monohydrate-anhydrous-conversion.7021/

martin@jangowski.de said:
1g Sodium Carbonate (anhydrous) = 1.17g Sodium Carbonate (monohydrated)

Martin

gainer said:
Washing soda is the traditional common name for the decahydrate. It may still be that in the UK. If so, you need 2.7 times as much of it as of the anhydrous, or 2.3 times as much as of the monohydrate. Otherwise, no difference.

jpohara · Aug 1, 2020

koraks said:
The anhydrate is neither cheaper nor more common. In fact it's relatively costly and since it's hygroscopic (it will absorb moisture from the air to form monohydrate), it's also not perfectly stable.
Are you sure you're not confusing anhydrous with decahydrate? The latter is indeed cheap and quite common as the cleaning soda sold in supermarkets.

Please let me know if I'm wrong, but I thought that the supermarket ("Arm & Hammer" brand) washing soda was the monohydrate, and that the decahydrate (crystalline) form is not typically seen commercially in the US. There is an MSDS on the A&H website if you dig for it which confirms this, if I recall correctly.

As an experiment, I took 100 g of supermarket washing soda and baked it in my toaster oven (I don't recall the temperature, but it was well below the temp at which sodium carbonate decomposes) for a couple of hours to cook the water out. The mass of the result was quite close to that predicted by my (distantly remembered) high school stoichiometry, given that the monohydrate has one molecule of H2O for each of carbonate.

The supermarket carbonate looks exactly like the monohydrate I had been buying from the usual suppliers (i.e., granular not crystalline), and works exactly the same when I use in my print developer. It is also much cheaper.

JPD · Aug 1, 2020

The decahydrate is difficult to use in photo chemistry as the crystals are big and lumped together, so you may need a hammer or knife to break the lumps apart and then a mortar and pestle to grind the crystals.

koraks · Aug 2, 2020

jpohara said:
Please let me know if I'm wrong, but I thought that the supermarket ("Arm & Hammer" brand) washing soda was the monohydrate, and that the decahydrate (crystalline) form is not typically seen commercially in the US.

You may be right; here in Europe, the decahydrate and monohydrate are sold next to each other, but the decahydrate sees more common. Either way, the anhydrate is not something I've ever encountered in the shops.

JPD said:
The decahydrate is difficult to use in photo chemistry as the crystals are big and lumped together

Depends on the form it comes in. I've had decahydrate in the form of the big lumps you describe, but also much finer granules. The latter dissolve fairly easily. As you point out, larger crystals can easily be ground to a finer granularity for easier processing.

GLS · Aug 2, 2020

Donald Qualls said:
Important clarification, since something up above could be read backward: if the recipe wants anhydrous, you have to use more monohydrate -- you're adding the required amount of sodium carbonate, plus the water of hydration it includes. So, if it's 15% water by mass, you need to use 115% as much by mass as the recipe calls for, not 83%.

Actually this isn't quite right either.

The molar mass of anhydrous sodium carbonate is 105.99, and the molar mass of the monohydrate is 124.00. So, a given mass of the monohydrate contains 105.99 / 124 = 0.855 times (rounded up) as much sodium carbonate as the same mass of the anhydrous material. Therefore, if a recipe calls for a mass of anhydrous sodium carbonate you will need 1 / 0.855 = 1.1695 (let's call it 117%) as much of the monohydrate, mass-wise.

Of course, unless the monohydrate is from a reputable supplier and has only just been opened you have no real way of knowing whether it is truly the monohydrate or contains significant quantities of higher hydrates.

Most of this is largely academic of course, as I really doubt most photographic formulas need to be that precise in terms of the absolute concentration of accelerant.

Donald Qualls · Aug 2, 2020

Notice the "if" in the last sentence you quoted? Point was and is that you need to increase the mass of hydrated chemical vs. a formula that specifies the anhydrous, rather than decreasing it as could be read in the post I replied to. Calculation of molar masses is left as an exercise.

FWIW, when I use sodium carbonate, I buy the laundry version and compensate for it being decahydrate. Cheap and easy to source, and photographically fine (because it's cheaper to make it to the same purity as soda ash for swimming pools and spas, technical grade at least, than to make multiple different grades).

GLS · Aug 2, 2020

Donald Qualls said:
Notice the "if" in the last sentence you quoted? Point was and is that you need to increase the mass of hydrated chemical vs. a formula that specifies the anhydrous, rather than decreasing it as could be read in the post I replied to. Calculation of molar masses is left as an exercise.

Yes I did, and I know that was the main point you were addressing. Now, did you notice the "isn't quite right" part of my post? If you are going to get specific about numbers and calculations in your post maybe don't get snippy when someone corrects them; this is a technical advise thread after all.

koraks · Aug 2, 2020

GLS said:
Of course, unless the monohydrate is from a reputable supplier and has only just been opened you have no real way of knowing whether it is truly the monohydrate or contains significant quantities of higher hydrates.

You'd have to test it, but I have decent confidence in the chemical industry. I don't think a significant error is likely to result from assuming that monohydrate is in fact monohydrate.

Wayne · Aug 2, 2020

Arm & Hammer washing soda is not decahydrate. That much I know for sure. I called them some years ago and they said it was monohydrate but in another thread here Rudeofus said the MSDS clearly indicates its anhydrous. Whichever it is, I'm 97.98% confident its not decahydrate, at least in the US.

JPD · Aug 3, 2020

Wayne said:
Arm & Hammer washing soda is not decahydrate. That much I know for sure. I called them some years ago and they said it was monohydrate but in another thread here Rudeofus said the MSDS clearly indicates its anhydrous. Whichever it is, I'm 97.98% confident its not decahydrate, at least in the US.

Ask them. They claim it's 100% Sodium carbonate, but that doesn't tell you if it's anhydrous, mono or decahydrate. I suspect it's technical grade monohydrate. That's pure enough for its purpose and the monohydrate is the most stable form. (Sodium carbonate is hygroscopic, and with that kind of packaging the anhydrous form would turn into monohydrate over time, and a decahydrate could lose water over time, and it forms lumps and would be difficult to pour from the packaging.) But the only way to know is by asking them and settle this question once and for all.

Donald Qualls · Aug 5, 2020

JPD said:
But the only way to know is by asking them and settle this question once and for all.

Well, other than doing your own dehydration testing. You'd weigh a sample, then heat it to drive off as much moisture as possible, followed by weighing again (ideally, before the sample can cool enough to pull moisture out of the air again).

Depending how strongly a species binds water, this might be easy or very difficult, but it does work with most compounds that pull water out of the air (like calcium chloride or ammonium nitrate).

However, based on properties, it seems very likely laundry soda is the monohydrate, and I've never had a problem using it with that assumption. Even in a homemade 2-bath C-41 color developer (Bath B is just sodium carbonate, potassium bromide, and water).

juan · Aug 6, 2020

Years ago, someone on Michael and Paula’s Azo forum contacted Arm & Hammer and was told their brand was monohydrate. I’ve used it under that assumption, too.

Wayne · Aug 7, 2020

JPD said:
Ask them. They claim it's 100% Sodium carbonate, but that doesn't tell you if it's anhydrous, mono or decahydrate. I suspect it's technical grade monohydrate. That's pure enough for its purpose and the monohydrate is the most stable form. (Sodium carbonate is hygroscopic, and with that kind of packaging the anhydrous form would turn into monohydrate over time, and a decahydrate could lose water over time, and it forms lumps and would be difficult to pour from the packaging.) But the only way to know is by asking them and settle this question once and for all.

I said I asked them and I said they told me its monohydrate. .

JPD · Aug 7, 2020

Wayne said:
I said I asked them and I said they told me its monohydrate. .

Yes, you did. But then you also mentioned that "Rudeofus said the MSDS clearly indicates its anhydrous" and said you were "97.98% confident its not decahydrate, at least in the US", so I went to their site to see if I could confirm once and for all that it is monohydrate. They only claimed it was 100% Sodium carbonate, so I checked their FAQ and didn't find more information there, so from there I went to their Contact page and started to write a question, but thought that someone from America should do that. We don't even have their products here in Europe. I should have worded my reply differently. Sorry.

juan said:
Years ago, someone on Michael and Paula’s Azo forum contacted Arm & Hammer and was told their brand was monohydrate. I’ve used it under that assumption, too.

I think we can say it is confirmed, then.

Wayne · Aug 9, 2020

JPD said:
Yes, you did. But then you also mentioned that "Rudeofus said the MSDS clearly indicates its anhydrous" and said you were "97.98% confident its not decahydrate, at least in the US", so I went to their site to see if I could confirm once and for all that it is monohydrate. They only claimed it was 100% Sodium carbonate, so I checked their FAQ and didn't find more information there, so from there I went to their Contact page and started to write a question, but thought that someone from America should do that. We don't even have their products here in Europe. I should have worded my reply differently. Sorry.

I think we can say it is confirmed, then.

That person on the Azo forum was probably me or someone reposting what I had posted. I wasn't super active there but I did go there now and then. years ago.

But I called them again to make sure, and the directed me to their product information. The ingredients are sodium carbonate anhydrous AND water. And I'm positive it isn't decahydrate so I'm comfortable in the belief that its monohydrate.

PinkPony · Jan 18, 2021

koraks said:
You may be right; here in Europe, the decahydrate and monohydrate are sold next to each other, but the decahydrate sees more common. Either way, the anhydrate is not something I've ever encountered in the shops.

Note to swedes (part of europe): "Nitor hushållssoda" seems to be 100% anhydrate sodium carbonate and is very cheap in photo chemistry quantities.
https://www.alfort.se/Global/Nitor/SDB/Upplagda 2015/NITOR HUSHÅLLSSODA clp.pdf

It says 100% CAS nr 497-19-8 which is anhydrate sodium carbonate.

I just mixed some FX-37 and had to check that I had anhydrate as I thought.

Sodium carbonate monohydrate vs anhydrous

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